1. Field of the Invention
This invention relates generally to asymmetrical interferometers, and more particularly to an interferometer wherein the areas of the Josephson junctions are not equal, and the magnetic lines of flux in the Josephson junctions are of substantially equal magnitude.
2. Description of the Prior Art
Asymmetrical interferometers are known to consist of inductive connections, or bridges, between Josephson junctions wherein the maximum Josephson currents in the Josephson junctions are of unequal magnitude. The maximum Josephson current of an interferometer I.sub.G is modulated with a control current I.sub.C by the magnetic field of a control line arranged above an inductive bridge. Asymmetrical interferometers are used instead of symmetrical interferometers having equal maximum Josephson currents, illustratively as storage cells for the nondestructive readout of individual flux quanta. Such a use is described in a published, nonprosecuted patent application (Offenlegungsschrift) No. 27 35 133, of H. Beha and W. Jutzi filed Aug. 4, 1977. In addition, asymmetrical interferometers have been used as logic gates in situations where the operating current must have particularly large tolerances. The use of an asymmetrical interferometer as a logic gate is described in the article "Asymmetric 2-Josephson Junction Interferometer As A Logic Gate," by H. Beha, Electronics Letters, Volume 13, pages 218 to 220, March, 1977.
Designers of asymmetrical interferometers usually optimize the devices under the simplifying assumption that the magnetic field of the control line in the interior of the Josephson contact is negligibly small. Under such assumptions, the interferometers are deemed to have point contacts.
The maximum Josephson current I.sub.G of interferometers which are deemed to have point contacts can be calculated relatively simply as a function of the control current I.sub.C and the number N of the flux quanta included in the interferometer loop. For each flux quantum state M, there is a closed area in the I.sub.G, I.sub.C -plane. Such a relationship is generally referred to as the "gate characteristic."
FIG. 1 is a plot of normalized gate current i.sub.G =IG/Io as a function of normalized control current i.sub.C =I.sub.C /I.sub.O. The regions of the different flux quantum states of interferometers having two hypothetical point contacts merge into each other by displacement along i.sub.c axis. For example, all of the maxima of the flux quantum states have the same height along the i.sub.c axis. FIG. 1, which is a normalized graphical representation of the maximum Josephson current I.sub.G plotted against the control current I.sub.C, illustrates the gate characteristic. As indicated, FIG. 1 shows the maximum Josephson current I.sub.G as a function of control current I.sub.C, i.e., I.sub.G =f(I.sub.C), where a parameter N is shown for an interferometer having two hypothetical point contacts, and with a ratio of maximum Josephson currents of 2/1 and with a characteristic phase: EQU .lambda.=2.pi.LI.sub.O /.phi..sub.O =2.pi.
In this characteristic phase equation, L is the inductance of the bridge between the contacts and I.sub.O is the maximum Josephson current of the smaller contact without external magnetic field. Also, .phi..sub.O =2.07 mVps is a flux quantum.
It is a problem with this analysis that if the interferometer is extremely miniaturized, the magnetic flux of the Josephson junctions can no longer be neglected in comparison with the flux of the interferometer ring, because of the finite resolution of the structuring processes. In contrast to interferometers having hypothetical point contacts, variations in the quantum states are especially pronounced and unfavorable in asymmetrical interferometers having Josephson junctions which have appreciably different contact lengths measured along the direction parallel to the control line. This results from the fact that a given magnetic control field will decrease the maximum Josephson current of the contact having the longer contact length noticeably more than the maximum Josephson current of the shorter contact. Thus, the inequality of the maximum Josephson currents, which is advantageous for producing large tolerances of the contacts, is reduced by the modulation.